Imaging device with varying optical signal

ABSTRACT

Example embodiments disclosed herein relate to an imaging device. The imaging device includes a multi-dimensional array of light sensitive elements, an illumination source that outputs an optical signal, an optical element that focuses the optical signal on the array, and an illumination controller. The illumination controller varies the output of the optical signal to control exposure at a location of the array.

BACKGROUND

A challenge exists to deliver quality and value to consumers, forexample, by providing various capabilities in imaging and printingdevices while maintaining cost effectiveness and output speed. Further,imaging and printing businesses may desire to enhance the functionalityof their devices. For example, such businesses may desire to provideenhanced image reproduction capability without requiring additionaleffort on the part of such consumers.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description references the drawings, wherein:

FIGS. 1 a-1 d illustrate a rolling reset and rolling readout of atwo-dimensional sensor array, as used in one example.

FIG. 2 is a graph illustrating a decrease of a percent of illuminationintensity as a function of the cosine of the angle off-center from anoptical axis, raised to the fourth power, as used in one example.

FIG. 3 shows the uncorrected illumination profile for a rectangulardocument captured by an 82 degree total field-of-view lens, prior tocorrection in one example.

FIG. 4 is a block diagram and example of an imaging device.

FIG. 5 is an example of an illumination level profile.

FIG. 6 shows an example of a corrected illumination profile at amulti-dimensional array of light sensitive elements for a rectangulardocument captured by an 82 degree total field-of-view lens.

FIG. 7 is another example of an illumination level profile.

FIG. 8 shows another example of a corrected illumination profile at amulti-dimensional array for a rectangular document captured by an 82degree total field-of-view lens.

FIG. 9 illustrates examples of some of the devices in which the imagingdevice of FIG. 4 may be used.

FIG. 10 is a block diagram and example of a method for use in an imagingdevice.

FIG. 11 is a block diagram and example of additional elements of themethod shown in FIG. 10.

DETAILED DESCRIPTION

Imaging devices ideally capture images and reproduce them as accuratelyas possible. These captured images can include things such asphotographs and scanned documents. However, realistic reproduction canbe difficult because of challenges and limitations associated with aparticular design.

For example, in some optical imaging systems, light from an image isfocused by optics onto a two-dimensional sensor array. Thetwo-dimensional sensor array is divided into rows of light sensitivepixels. A rolling reset first proceeds through the two-dimensionalsensor array and successively resets each row. After an appropriateexposure time has elapsed, a rolling readout operation proceeds throughthe sensor array to capture the exposure value for the pixels in thatarray. An image is then constructed from these exposure values.

For example, as shown in FIG. 1 a, a rolling reset starts at row one 10of two-dimensional sensor 20, at time zero (t=0), and then proceeds downone row per time period in the direction of arrow 30, as shown in FIG. 1b. The time period is set to be the amount of time the system requiresto read one row. In this example, the exposure time is set to five suchthat row one 10 is read out by the system after five time periods, asshown in FIG. 1 c. Subsequently, row two 40 is read out at time six(t=6), as shown in FIG. 1 d. In a typical system, all rows in the sensorarray have an identical exposure duration using a combination of therolling shutter and the subsequent rolling readout.

In such optical imaging systems, the collected light is lower forportions of an image off-center from the optical axis. The illuminationintensity decreases as a function of the cosine of the angle off-centerfrom the optical axis, raised to the fourth power. FIG. 2 illustratesthis effect graphically where the percent illumination 50 is graphed asa function of the angle theta in degrees off-center from the opticalaxis 60.

This “cosine-four impact” causes the collected light and correspondingsignal-to-noise ratio to be significantly lower for the outer portionsof the image because illumination is controlled to prevent the centralportion of an image from being saturated. This results in low imagequality at the outer edges of the image. For systems using atwo-dimensional imaging sensor, the signal-to-noise at the corners ofthe image can be more than three-times lower than the signal-to-noise inthe center of the image.

In addition, non-uniformity of the illumination source can furtherreduce the signal-to-noise at the corners of the image. This occursbecause in many illumination sources the output signal or intensity istypically brighter towards the center of an illuminated area and isreduced in intensity towards the edges of the illuminated area.

FIG. 3 shows the uncorrected illumination profile 70 for a rectangulardocument captured by an 82 degree total field-of-view lens. For auniform target, the illumination intensity or level 75 at the corners 80of the image is approximately 32% of the illumination intensity at thecenter 90 of the optical field. FIG. 3 illustrates the results for animage sensor using a 44×33 grid (44 columns 100 and 33 rows 110) whichcorresponds to a 4:3 aspect ratio used in some image sensors.

A solution to this “cosine-four impact” is to apply analog or digitalgain to the outer portions of the image to increase the signal andnormalize the image. However, such a gain technique increases both thesignal and the noise such that the signal-to-noise ratio of the outerportions is typically very poor. Alternatively, a system can capturemultiple images with different illumination levels and construct acomposite image. However, this method is computationally intensive andit is difficult to achieve proper uniformity and linearity in the image.It can also add cost due to the additional needed computing capabilityand can lower the speed at which images can be produced.

To optimize the signal-to-noise in the image, the illumination of thecaptured image should be as uniform as possible over the image field ofthe sensor. An example of an imaging device 120 designed with this goalin mind is shown in the block diagram of FIG. 4. Imaging device 120includes a multi-dimensional array of light sensitive elements 130, anillumination source 140 that outputs an optical signal 150, an opticalelement 160, and an illumination controller 170. As shown in FIG. 4,illumination source 140 may include one or more light-emitting diodes(LEDs) 145. Multi-dimensional array 130 includes a plurality of rows 180and columns 190 of light sensitive elements that are arranged so as tobe positioned relative to a central location 200. In this particularexample, array 130 includes thirty-three (33) rows 180 and forty-four(44) columns 190.

Illumination source 140 generates an optical signal 150 that illuminatesobject 155. Optical element 160 is designed to image object 155 which isilluminated by optical signal 150 on multi-dimensional array of lightsensitive elements 130. Optical element 160 can be a variety of designsand include one or more lenses, mirrors, or a combination of the two.Illumination controller 170 is designed to vary an output of opticalsignal 150 of illumination source 140 to control exposure at one or morelocations of array 130, as generally indicated by double arrow 210. Forexample, illumination controller 170 may vary the output of opticalsignal 150 based on distance from central location 200. As anotherexample, illumination controller 170 may vary the output of opticalsignal 150 based upon a location of a row 180 within array 130 or basedupon a location of a column 190 within array 130.

Imaging device 120 utilizes a rolling shutter that consists of twosynchronized events, a rolling reset and a rolling readout, as describedearlier in FIGS. 1 a-1 d, both of which occur on multi-dimensional array130 of light sensitive elements. First the rolling reset proceedsthrough array 130 and successively resets each of the rows 180 to aninitial value. After the appropriate exposure time has elapsed, therolling readout operation proceeds through each of the rows 180 of array130. Illumination controller 170 synchronizes optical signal 150 ofillumination source 140 with the rolling shutter such that the output ofoptical signal 150 is increased while exposing rows 180 or columns 190further away from central location 200 and decreased during exposure ofcentral location 200. This technique allows the illumination level to beincreased for the outer portions of the image and reduces the“cosine-four impact”, discussed above.

Illumination controller 170 can be of a variety of designs and include aprocessor that executes instructions stored on a non-transitorycomputer-readable medium (CRM) 220, as generally indicated by doublearrow 230. Computer-readable medium 220 can include any type of memorydevice or combination thereof, such as a hard drive, read only memory(ROM), random access memory (RAM), flash drive, etc.

For example, because multi-dimensional array of light sensitive elementsis divided into thirty-three (33) rows 180, the illumination level ofoptical signal 150 of illumination source 140 for the first row can beset to 1.65 times the illumination level for the row at central location200. The illumination level of optical signal 150 of illumination source140 for the second row is then set to 1.56 times the illumination levelfor the row at central location 200. The illumination level of opticalsignal 150 of illumination source 140 is thus selected, as illustratedin FIG. 5, This helps optimize the illumination for each row to get anominal exposure. The nominal exposure is the amount of light thatincreases the center pixel of a row to be substantially similar to thepixels in the optical center of the field for a uniform target.

If illumination controller 170 causes illumination source 140 to producean optical signal 150 across rows 180 of array 130 with the illuminationlevel profile shown in FIG. 5, the illumination at multi-dimensionalarray of light sensitive elements 130 is improved to the level shown ingraph 240 of FIG. 6. With this implementation, the illumination level250 at edges 260 is now the worst case as opposed to the corners 80 inthe non-optimized system shown in FIG. 3. For corners 270 of the imagerepresented by graph 240 shown in FIG. 6, the illumination ratioincreases from approximately 32% to approximately 53%. This increasesthe signal-to-noise ratio by approximately 1.6× for corners 270. Theworst case illumination is now located at edges 260 in row 17 and isapproximately 46% of the central location 280 illumination level 250.The illumination level 250 remains substantially the same for centrallocation 280 because the level of optical signal 150 is selected to besubstantially equal to the non-optimized system illustrated in FIG. 3.

As an alternative example, illumination controller 170 can causeillumination source 140 to proceed, in an orthogonal direction to thatillustrated in FIGS. 5 and 6, to produce an optical signal 150 acrossthe forty-four (44) columns 190 of array 130. In this implementation,the rolling shutter is designed to move through the columns 190 of array130. This approach improves the illumination level to an even greaterextent than that illustrated in FIGS. 5 and 6. FIG. 7 shows anillumination level profile 290 utilizing this approach for each of theforty-four (44) columns 190 of array 130. FIG. 8 shows a graph 300 ofthe illumination level at multi-dimensional array of light sensitiveelements 130 that results from the utilization of the illuminationprofile illustrated in FIG. 7 for the forty-four (44) columns 190 ofarray 130.

As can be seen in FIG. 8, the minimum illumination level 310 for thecorners 320 of the image is approximately 70% versus approximately 32%for the non-optimized system shown in FIG. 3. This corresponds to anapproximate 2.16× increase in signal-to-noise ratio of the corners 320.The illumination level remains substantially the same for the centrallocation 330 because the level of optical signal 150 is selected to besubstantially equal to the non-optimized system shown in FIG. 3.However, the minimum illumination level 310 is approximately 61% foredges 340 of graph 300 for this optimized system versus approximately32% for corners 80 of the non-optimized system illustrated in FIG. 3.This means the worst case signal-to-noise ratio is improved byapproximately 90% for this implementation.

The illumination level profile can be different from those used in theexamples provided above and discussed with respect to FIGS. 5-8. Theparticular illumination level profile is selected to optimize theillumination level at multi-dimensional array of light sensitiveelements 130 based on the particular characteristics of an imagingdevice (e.g., the field-of-view, geometry of the multi-dimensional arrayof light sensitive elements 130, etc.). These illumination profiles canbe determined in a variety ways and derived computationally as needed orstored in a look-up table on computer-readable medium 220.

As shown in FIG. 9, there are several applications for the imagingdevice 120 illustrated in FIG. 4. For example, it can be used in acamera 345, printing device 350 and a scanner 360. Although notillustrated, it is to be understood that other applications are alsopossible.

An example of a method 370 for use in an imaging device 120 isillustrated in FIG. 10. Method 370 begins by illuminating an object withan optical signal from an illumination source, as shown by block 380.Next, method 370 proceeds by starting a rolling shutter moving across amulti-dimensional array of light sensitive elements and enabling a firstportion of the array to collect light provided by the optical signalfrom the illumination source, as shown by block 390 in FIG. 10. Next,method 370 records the signal from this first portion of the array, asshown in block 400 in FIG. 10. Method 370 adjusts a level of the opticalsignal from the illumination source so that the level is appropriate fora next portion of the array, as shown by block 410 of FIG. 10. The levelof the optical signal at the first portion of the multi-dimensionalarray of light sensitive elements may be increased relative to the levelof the optical signal at the second portion of the multi-dimensionalarray of light sensitive elements.

Next, method 370 moves the rolling shutter to a next portion of themulti-dimensional array of light sensitive elements, as generallyindicated by block 420 in FIG. 10. Next, method 370 records the signalfor the next portion of the array, as shown by block 430 in FIG. 10.Method 370 then determines if all portions of the array have been read,as generally indicated by block 435 in FIG. 10. If all portions havebeen read, method 370 ends. If not, then method 370 returns back toblock 410 and continues, as indicated in FIG. 10.

Method 370 may also construct an image from the signals recorded fromall the portions of the array, as shown by block 440 in FIG. 11.Finally, method 370 may conclude by printing the constructed image, asindicated by block 450 of FIG. 11.

Although several examples have been described and illustrated in detail,it is to be clearly understood that the same are intended by way ofillustration and example only. These examples are not intended to beexhaustive or to limit the invention to the precise form or to theexemplary embodiments disclosed. Modifications and variations may wellbe apparent to those of ordinary skill in the art. For example, therolling reset and/or the rolling readout operation can be performed at avarying speed, rather than a uniform one. As an additional example, theillumination source 140 may include other optical signal sources such asone or more bulbs, rather one or more LEDs 145. The spirit and scope ofthe present invention are to be limited only by the terms of thefollowing claims.

Additionally, reference to an element in the singular is not intended tomean one and only one, unless explicitly so stated, but rather means oneor more. Moreover, no element or component is intended to be dedicatedto the public regardless of whether the element or component isexplicitly recited in the following claims.

1. An imaging device, comprising: a multi-dimensional array of light sensitive elements that includes a central location; an illumination source that outputs an optical signal; an optical element that focuses the optical signal of the illumination source on the multi-dimensional array of light sensitive elements; and an illumination controller that varies an output of the optical signal of the illumination source to control an exposure at a location of the multi-dimensional array based upon distance from the central location.
 2. The imaging device of claim 1, wherein the multi-dimensional array of light sensitive elements includes a plurality of rows and the illumination controller varies the output of the optical signal based upon a location of the row in the multi-dimensional array.
 3. The imaging device of claim 1, wherein the multi-dimensional array of light sensitive elements includes a plurality of columns and the illumination controller varies the output of the optical signal based upon a location of the column in the multi-dimensional array.
 4. The imaging device of claim 1, further comprising a reset operation that proceeds through the multi-dimensional array of light sensitive elements and a readout operation that proceeds through the multi-dimensional array of light sensitive elements.
 5. The imaging device of claim 4, wherein one of the reset operation and the readout operation is performed at a varying speed.
 6. The imaging device of claim 1, further comprising a non-transitory computer-readable storage medium including a look-up table of values representative of an illumination profile, and wherein the illumination controller varies the output of the optical signal of the illumination source based on the look-up table of values.
 7. The imaging device of claim 1, wherein the illumination controller increases the output of the optical signal as a distance from the central location increases.
 8. The imaging device of claim 1, wherein the multi-dimensional array of light sensitive elements includes an inner portion and an outer portion and the illumination controller increases the output of the illumination source for the outer portion relative to the output of the illumination source for the inner portion.
 9. The imaging device of claim 1, further comprising one of a printing device, camera, and scanner.
 10. A non-transitory computer-readable storage medium storing instructions that, when executed by a processor, cause the processor to: illuminate a first portion of a multi-dimensional array of light sensitive elements with an optical signal from an illumination source; illuminate a second portion of the multi-dimensional array of light sensitive elements with the optical signal from the illumination source; and control a level the optical signal from the light source so that the level is different at the first portion of the multi-dimensional array of light sensitive elements than the level at the second portion of the multi-dimensional array of light sensitive elements.
 11. The non-transitory computer-readable medium of claim 10, wherein the level of the optical signal at the first portion of the multi-dimensional array of light sensitive elements and the level of the optical signal at the second portion of the multi-dimensional array of light sensitive elements are each selected to compensate for light falloff within the optical field.
 12. The non-transitory computer-readable medium of claim 10, wherein the level of the optical signal is increased during illumination of an edge of the optical field and the level of the optical signal is decreased during illumination of a center of the optical field.
 13. The non-transitory computer-readable medium of claim 10, further comprising stored instructions that, when executed by a processor, cause the processor to record the light collected by a first set of light sensitive elements located adjacent the first portion of the multi-dimensional array of light sensitive elements.
 14. The non-transitory computer-readable medium of claim 13, further comprising stored instructions that, when executed by a processor, cause the processor to construct an image from the light collected by the first set of light sensitive elements located adjacent the first portion of the multi-dimensional array of light sensitive elements.
 15. The non-transitory computer-readable medium of claim 14, further comprising stored instructions that, when executed by the processor to print an image constructed from the light collected by the multi-dimensional array of light sensitive elements.
 16. A method for use in an image recording apparatus, comprising: illuminating a first portion of a multi-dimensional array of light sensitive elements with an optical signal from an illumination source; illuminating a second portion of the multi-dimensional array of light sensitive elements with the optical signal from the illumination source; and adjusting a level of the optical signal from the illumination source so that the level is different at the first portion of the multi-dimensional array of light sensitive elements than the level of the optical signal at the second portion of the multi-dimensional array of light sensitive elements.
 17. The method of claim 16, further comprising increasing the level of the optical signal at the first portion of the multi-dimensional array of light sensitive elements relative to the level of the optical signal at the second portion of the multi-dimensional array of light sensitive elements.
 18. The method of claim 16, further comprising recording light collected by a first set of light sensitive elements located adjacent the first portion of the multi-dimensional array of light sensitive elements.
 19. The method of claim 18, further comprising constructing an image from the light collected by the first set of light sensitive elements located adjacent the first portion of the multi-dimensional array of light sensitive elements.
 20. The method of claim 19, further comprising printing an image constructed from the light collected by the multi-dimensional array of light sensitive elements. 